RF shielding and RF shielded rooms/shelters for military and other government uses have been in use for some time. The United States military provides specifications for such shelters at Mil-Std-188-125-1 and Mil-Std-188-125-2 (HEMP Shelters), which are incorporated herein by reference (see web site http://www.everyspec.com/MIL-STD/MIL-STD+(0100+−+0299)/MIL-STD-188—125—1−1—4470/ and http://www.everyspec.com/MIL-STD/MIL-STD+(0100+−+0299)/MIL-STD-188-125-2(NOTICE—1)—4473/).
Historically, these government specifications have been met exclusively through use of a so-called “knife edge” door, i.e., a door design in which an RF seal is created where the door joins the door frame by a brass knife edge on the door enters a channel on the door frame. An example of this type of door is shown at http://www.etslindgren.com/pdf/iSKE.pdf; http://www.jaycor.com.
This knife-edge door design has numerous disadvantages, however. Because the knife and channel are made of brass, corrosion occurs and creates non conductive zinc and copper oxides. This oxidation in turn decreases the RF shielding effectiveness of the door seal.
In addition, the channel traps water dirt and contaminants, whereby shielding performance degrades exponentially. Also, the channel is extremely difficult to clean. Typically, cleaning requires removal of the fingerstock in the channel (that is, brass receiving “fingers” that help create an electrical seal with the knife edge in the channel. The fingerstock, when removed, often gets damaged and cannot be reused. Also, all corrosion has to be removed from channel and knife edge surfaces, which is difficult. A conductive lubricant can be used on the brass surfaces to slow corrosion. However, the silicone lubricant traps and holds dirt and dust particles reducing shielding effectiveness.
In addition, water freezes in channel rendering the door inoperable in cold climates. Moreover, the knife edge can wear below serviceable limits in dry sandy environments requiring replacement of entire door within 5 years. In this event, the HEMP enclosure has to be removed from service until repaired
The knife edge design also presents disadvantages because the doors using this design cannot be opened or closed without mechanical assistance. Large lever and cam mechanisms are required to open and close the knife edge door. Appreciable wear on the fingerstock and knife edge occurs because of this mechanical opening action. There are two conditions that make the door difficult to operate: 1—the beryllium copper fingerstock are heat treated, or tempered, to make them springy. This process also hardens them. When the surface of the fingerstock begins to wear and become microscopically abraded, it digs in and grabs the softer brass knife edge requiring more and more effort to operate the lever mechanism. This can be visually confirmed by the grooves that each of the fingers eventually wears into the brass knife edge; 2—the lever mechanism only unseats the knife edge on the strike side of the door requiring the operator to manually pull the door's knife edge completely out of the channel and fingerstock at the top, bottom and hinge side and to push the door in until the lever mechanism can be engaged.
Very high maintenance is required for the knife edge door design. In particular, weekly flushing of the channel with solvents is required to remove loose dirt. In addition, weekly lubrication with conductive lubricant of the fingerstock in the channel is recommended. For the reasons discussed above, monthly or quarterly replacement of fingerstock occurs—with associated down time—based upon usage of the knife edge door. Fingerstock replacement requires special tools and takes approximately 1 hour. Moreover, monthly or quarterly lubrication of the mechanical operating mechanism is required based upon usage. Other repair needs include repair and replacement of worn beyond limits parts in the operating mechanism. Finally, the operating mechanism shaft seal needs periodic replacement to maintain shielding effectiveness.
Additional problems with the knife edge design arise because the brass knife edge can be bent causing misalignment—which makes the door difficult if not impossible to operate and causes a loss of shielding effectiveness. Similarly, the knife edge at the sill cannot be stepped on as damage will occur. The sill must be protected by a steel plate or wood ramp of sufficient strength if furniture, fixtures or supplies need to be wheeled or carted through the door. The knife edge design does not meet ADA door sill height requirements of less than ½″, and commonly requires a 2-3″ step over.